Person support apparatuses with load cell error detection

ABSTRACT

A person support apparatus, such as a bed, stretcher, recliner, cot, or the like, includes a frame, a plurality of load cells, a support surface supported by the load cells, a detection circuit, and a controller. The controller determines if any of the load cells are in an error state based upon information from the detection circuit. If the load cells include memory having calibration data stored therein, the controller communicates with the memory and uses the calibration data to determine an amount of weight supported on the surface. The detection circuit may include one or more Wheatstone bridges wherein the controller monitors voltages between midpoints of the Wheatstone bridges. The load cells may include an activation lead that is monitored by the detection circuit and a sensor lead that is used by the controller to determine an amount of weight supported on the patient support apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationSer. No. 62/186,464 filed Jun. 30, 2015, by inventors Marko Kostic etal. and entitled PERSON SUPPORT APPARATUSES WITH LOAD CELLS, thecomplete disclosure of which is hereby incorporated herein by reference.

BACKGROUND

The present disclosure relates to person support apparatuses, such asbeds, cots, stretchers, operating tables, recliners, or the like. Morespecifically, the present disclosure relates to person supportapparatuses that include load cells.

Existing hospital beds and/or stretchers often include a load cellsystem that is used to detect the weight of an occupant of the bed orstretcher, and/or that is used as an exit detection system. Whenfunctioning as a scale system, the outputs of the load cells are readand a weight of the occupant is detected. When functioning as an exitdetection system, the outputs of the load cells are read and used todetect when a patient has exited the bed or stretcher, or when a patientmay be about to exit the bed or stretcher.

Often, conventional load cell systems include a processor ormicrocontroller that monitors the outputs of the load cells and detectsan error state if one or more of the load cells report force values thatare outside of an expected range. For example, if an individual loadcell is designed in the person support apparatus to sense weightsbetween 0 and 500 pounds, and that load cell outputs a voltagecorresponding to 700 pounds, the processor or microcontroller interpretsthis as an error state.

SUMMARY

According to various embodiments, the present disclosure provides aperson support apparatus having an improved load cell system that isconfigured to better detect error states of the load cells. In someembodiments, the detected error states are due to the load cell notbeing electrically connected, or improperly connected, to the processoror microcontroller that reads the outputs of the load cells. In someembodiments, the improved load cell system is also or alternativelyadapted to automatically detect error states relating to an activationvoltage source that supplies power to the load cells. The automaticerror state detection of the improved load cell system is better able todetect error states of the load cells than previous load cell systems.Further, in some embodiments, the load cells are configured to includeload cell-specific and/or person support apparatus-specific calibrationdata in a memory integrated into the load cells. This stored calibrationdata enables a defective load cell to be replaced without requiringrecalibration of the replacement load cell after it is installed in theperson support apparatus.

According to one embodiment, a person support apparatus is provided thatincludes a frame, a plurality of load cells, a support surface, adetection circuit, and a controller. The load cells are supported by theframe, and the support surface is supported by the load cells such thata weight of the occupant is detectable by the load cells when theoccupant is positioned on the support surface. The detection circuit isin communication with the load cells, and the controller is incommunication with the detection circuit. The controller determines ifany of the plurality of load cells are in an error state based uponinformation from the detection circuit, and outputs an error signal ifone or more of the plurality of load cells are in the error state.

The error state may comprise one or more of the plurality of load cellsnot being electrically coupled to an activation power source, or it maycomprise one or more of the load cells malfunctioning while the loadcells are electrically coupled to the activation power source.

In some embodiments, each load cell includes a first set of leads forpowering the load cell and a second set of leads for outputting a signalthat varies as a function of a physical load applied to the load cell.The detection circuit is in electrical communication with the first setof leads but not the second set of leads. The detection circuit detectschanges in a total amount of electrical current supplied to all of thefirst sets of leads from the activation power source. Further, thecontroller determines that one or more of the plurality of load cellsare not electrically coupled to the activation power source, or areotherwise malfunctioning, if the total amount of electrical currentsupplied to all of the first sets of leads is outside of a predeterminedrange. The predetermined range may be a function of the number of loadcells in the plurality of load cells, as well as a function of the typeof load cells. The predetermined range has a magnitude greater than amagnitude of an expected change in the current supplied to the first setof leads when no load is applied to the load cells and when a maximumrated load is applied to the load cells.

The detection circuit, in some embodiments, identifies individual onesof the plurality of load cells that are present and eithermalfunctioning or functioning correctly. The controller identifies anindividual one of the plurality of load cells as being present andfunctioning normally if an amount of electrical current supplied to thefirst set of leads to the individual one of the plurality of load cellsfalls within a predetermined range.

The detection circuit also includes, in some embodiments, a plurality ofswitches controlled by the controller. The switches are arranged inseries with one or more of the plurality of load cells.

In some embodiments, a user interface is provided that is adapted toallow a user to inform the controller when no load is supported on thesupport surface. The controller opens or closes at least one of theswitches when no load is supported on the support surface. Thecontroller uses the opening and closing of at least one of the switchesto distinguish between one or more of the load cells being absent(and/or malfunctioning) and an improper activation voltage being appliedto the plurality of load cells from the activation power source.

The controller communicates with the second set of leads of each of theplurality of load cells and the controller determines an amount ofweight applied to the support surface based upon voltage changes in thesecond set of leads of each of the plurality of load cells, in someembodiments.

According to another embodiment, a person support apparatus is providedthat includes a frame, first and second load cells supported by theframe, a support surface, first and second activation leads, first andsecond sensor leads, an activation voltage source, a controller, and adetection circuit. The support surface is adapted to support thereon anoccupant of the person support apparatus, and it is supported by atleast the first and second load cells such that a weight of the occupantis at least partially detectable by the first and second load cells whenthe occupant is positioned on the support surface. The first and secondactivation leads are coupled to the first and second load cells,respectively. The first and second sensor leads are also coupled to thefirst and second load cells, respectively. The activation voltage sourceis adapted to provide a substantially constant voltage to the first andsecond activation leads. The controller determines an amount of weightapplied to the support surface based on voltage changes in the first andsecond sensor leads. The detection circuit communicates with theactivation voltage source, the first and second activation leads, andthe controller; and the controller is adapted to detect if the firstload cell is in an error state based upon information from the detectioncircuit.

The error state includes an electrical disconnection of the first loadcell from the controller, as well as the malfunctioning of the firstload cell while electrically connected.

In some embodiments, the controller monitors a total amount ofelectrical current flowing from the activation voltage source to thefirst and second activation leads. The controller determines that thefirst load cell is in an error state if the total amount of electricalcurrent flowing from the activation voltage source to the first andsecond activation leads moves outside of a predetermined range.

The detection circuit includes a first switch in series with the firstload cell and a second switch in series with the second load cell, in atleast some embodiments. The controller controls the first and secondswitches to distinguish between the first load cell being in the errorstate and the second load cell being in the error state. In otherembodiments, the controller uses the first and second switches todistinguish between the first load cell being in an error state and theactivation voltage source being in an error state. The error state ofthe activation voltage source includes a substantially non-constantvoltage being supplied by the activation voltage source to the first andsecond activation leads, or a substantially constant voltage having anincorrect value being supplied by the activation voltage source.

In some embodiments, the first load cell includes a first Wheatstonebridge, the second load cell includes a second Wheatstone bridge, andthe first and second Wheatstone bridges are part of a third Wheatstonebridge. When so arranged, the controller monitors a voltage betweenmidpoints of the third Wheatstone bridge when no load is supported onthe support surface, and the controller issues an error signal if thevoltage between midpoints of the third Wheatstone bridge exceeds athreshold level when no load is supported on the support surface.

In still other embodiments, the controller stores first calibration datafor the first load cell and second calibration data for the second loadcell in a memory on board the person support apparatus. The firstcalibration data includes a first subset of calibration data generatedbefore the first load cell is installed in the person support apparatusand a second subset of calibration data generated after the first loadcell is installed in the person support apparatus. The secondcalibration data includes a third subset of calibration data generatedbefore the second load cell is installed in the person support apparatusand a fourth subset of calibration data generated after the second loadcell is installed in the person support apparatus.

The controller is further adapted to use the second subset ofcalibration data when a replacement load cell is installed in the personsupport apparatus, wherein the replacement load cell replaces the firstload cell. The controller uses the second subset of calibration data tocalibrate the replacement load cell. The controller calibrates thereplacement load cell without requiring any calibration measurements tobe taken regarding the replacement load cell after the replacement loadcell is installed in the person support apparatus.

In some embodiments, the first subset of calibration data is stored in afirst memory physically coupled to the first load cell and the thirdsubset of calibration data is stored in a second memory physicallycoupled to the second load cell. The controller reads the first andthird subsets of calibration data from the first and second memories,respectively, and stores them in the memory on board the person supportapparatus.

According to another embodiment, a person support apparatus is providedthat includes a frame, first and second load cells, a support surface,and a controller. The first and second load cells are supported by theframe and include first and second Wheatstone bridges, respectively. Thefirst and second Wheatstone bridges are arranged as part of a thirdWheatstone bridge. The support surface is supported by at least thefirst and second load cells such that a weight of the occupant is atleast partially detectable by the first and second load cells when theoccupant is positioned on the support surface. The controllercommunicates with the first, second, and third Wheatstone bridges and isadapted to monitor voltages between midpoints of each of the first,second, and third Wheatstone bridges.

The controller uses the voltage between the midpoints of the firstWheatstone bridge and the voltage between the midpoints of the secondWheatstone bridge to determine an amount of weight applied to thesupport surface. The controller uses the voltage between the midpointsof the third Wheatstone bridge to determine if an error state exists forat least one of the first and second load cells. In some embodiments,the controller also, or alternatively, uses the voltage between themidpoints of the third Wheatstone bridge to determine if an error stateexists for at least one of the first and second load cells when no loadis supported on the support surface.

The person support apparatus may also include a detection circuit inelectrical communication with the first and second load cells and thecontroller, wherein the controller is further adapted to determine if anerror state exists for at least one of the first and second load cellsbased upon information from the detection circuit. An activation voltagesource may be coupled to endpoints of the first and second Wheatstonebridges, wherein the detection circuit monitors a total amount ofelectrical current flowing from the activation voltage source to theendpoints of the first and second Wheatstone bridges. The controllerdetermines that at least one of the first and second load cells is in anerror state if the total amount of electrical current flowing from theactivation voltage source to the endpoints moves outside of apredetermined range.

According to yet another embodiment, a person support apparatus isprovided that includes a frame, a support surface, a load cell memory, aperson support apparatus memory, and a controller. The support surfaceis supported by the load cell such that a weight of the occupant is atleast partially detectable by the load cell when the occupant ispositioned on the support surface. The load cell memory is integratedwithin the load cell and includes first calibration data regarding theload cell stored therein. The first calibration data is generated beforethe load cell is installed in the person support apparatus. The personsupport apparatus memory contains second calibration data regarding theload cell that is generated after the load cell is installed in theperson support apparatus. The controller communicates with the loadcell, the load cell memory, and the person support apparatus memory. Thecontroller uses the first calibration data and the second calibrationdata to determine an amount of weight applied to the support surface

The controller uses the second calibration data to calibrate areplacement load cell when the replacement load cell is installed in theperson support apparatus that replaces the load cell. The controllerdoes not use the first calibration data when the replacement load cellis installed in the person support apparatus. The controller may alsocalibrate the replacement load cell without requiring any calibrationmeasurements to be taken regarding the replacement load cell after thereplacement load cell is installed in the person support apparatus.

According to yet another embodiment, a person support apparatus isprovided that includes a frame, a plurality of sensors supported by theframe, a structure supported by the sensors, a detection circuit, and acontroller. The structure is supported by the sensors in such a way thatthe sensors are able to detect a force exerted on the structure by aperson. The detection circuit is in communication with the sensors andthe controller is in communication with the detection circuit. Thecontroller determines if any of the plurality of sensors are in an errorstate based upon information from the detection circuit.

In some embodiments, the sensors are load cells.

According to yet another embodiment, a person support apparatus isprovided that includes a frame, first and second sensors support by theframe, a structure, and a controller. The first and second sensors areincluded within first and second Wheatstone bridges, respectively, andthe first and second Wheatstone bridges are included within a thirdWheatstone bridge. The structure is supported by the first and secondsensors in such a way that the first and second sensors are able todetect a force exerted on the structure by a person. The controllercommunicates with the first, second, and third Wheatstone bridges, andis adapted to monitor voltages between midpoints of each of the first,second, and third Wheatstone bridges.

The controller uses the voltage between the midpoints of the firstWheatstone bridge and the voltage between the midpoints of the secondWheatstone bridge to determine a magnitude of the force exerted on thestructure, and the controller uses the voltage between the midpoints ofthe third Wheatstone bridge to determine if an error state exists for atleast one of the first and second sensors.

In any of the person support apparatuses described herein, the personsupport apparatus may be one of a bed, a recliner, a cot, and astretcher. Further, in some embodiments, at least one of the load cellsis used to detect a user-applied force that is used by the controller tocontrol a propulsion system on-board the person support apparatus.

Before the various embodiments disclosed herein are explained in detail,it is to be understood that the claims are not to be limited to thedetails of operation or to the details of construction and thearrangement of the components set forth in the following description orillustrated in the drawings. The embodiments described herein arecapable of being practiced or being carried out in alternative ways notexpressly disclosed herein. Also, it is to be understood that thephraseology and terminology used herein are for the purpose ofdescription and should not be regarded as limiting. The use of“including” and “comprising” and variations thereof is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items and equivalents thereof. Further, enumeration may beused in the description of various embodiments. Unless otherwiseexpressly stated, the use of enumeration should not be construed aslimiting the claims to any specific order or number of components. Norshould the use of enumeration be construed as excluding from the scopeof the claims any additional steps or components that might be combinedwith or into the enumerated steps or components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a person support apparatus according toa first embodiment;

FIG. 2 is a perspective view of a litter and a pair of lift headerassemblies with load cells of the person support apparatus of FIG. 1;

FIG. 3 is a block diagram of a load cell system that may be incorporatedinto the person support apparatus of FIG. 1, as well as other personsupport apparatuses;

FIG. 4 is a block diagram of a first embodiment of a detection circuitthat may be used with the load cell system of FIG. 3;

FIG. 5 is a block diagram of a second embodiment of a detection circuitthat may be used with the load cell system of FIG. 3;

FIG. 6 is a block diagram of a third embodiment of a detection circuitthat may be used with the load cell system of FIG. 3;

FIG. 7 is a block diagram of a fourth embodiment of a detection circuitthat may be used with the load cell system of FIG. 3;

FIG. 8 is a block diagram of a fifth embodiment of a detection circuitthat may be used with the load cell system of FIG. 3;

FIG. 9 is a block diagram of a sixth embodiment of a detection circuitthat may be used with the load cell system of FIG. 3;

FIG. 10 is a block diagram of a load cell system according to a secondembodiment that may be incorporated into the person support apparatus ofFIG. 1, as well as other person support apparatuses; and

FIG. 11 is a block diagram of a load cell system according to a thirdembodiment that may be incorporated into the person support apparatus ofFIG. 1, as well as other person support apparatuses.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An illustrative person support apparatus 20 according to a firstembodiment is shown in FIG. 1. Although the particular form of personsupport apparatus 20 illustrated in FIG. 1 is a bed adapted for use in ahospital or other medical setting, it will be understood that personsupport apparatus 20 could, in different embodiments, be a cot, astretcher, a gurney, a recliner, an operating table, a residential bed,or any other structure capable of supporting a person, whetherstationary or mobile and/or whether medical or residential.

In general, person support apparatus 20 includes a base 22 having aplurality of wheels 24, a pair of lifts 26 supported on the base, alitter frame 28 supported on the lifts 26, and a support deck 30supported on the litter frame 28. Person support apparatus 20 furtherincludes a headboard 32, a footboard 34, and a plurality of siderails36. Siderails 36 are all shown in a raised position in FIG. 1 but areeach individually movable to a lower position in which ingress into, andegress out of, person support apparatus 20 is not obstructed by thelowered siderails 36.

Lifts 26 are adapted to raise and lower litter frame 28 with respect tobase 22. Lifts 26 may be hydraulic actuators, electric actuators, or anyother suitable device for raising and lowering litter frame 28 withrespect to base 22. In the illustrated embodiment, lifts 26 are operableindependently so that the tilting of litter frame 28 with respect tobase 22 can also be adjusted. That is, litter frame 28 includes a headend 38 and a foot end 40, each of whose height can be independentlyadjusted by the nearest lift 26. Person support apparatus 20 is designedso that when an occupant lies thereon, his or her head will bepositioned adjacent head end 38 and his or her feet will be positionedadjacent foot end 40.

Litter frame 28 provides a structure for supporting support deck 30,headboard 32, footboard 34, and siderails 36. Support deck 30 provides asupport surface for a mattress (not shown in FIG. 1), or other softcushion, so that a person may lie and/or sit thereon. The top surface ofthe mattress or other cushion forms a support surface for the occupant.Support deck 30 is made of a plurality of sections, some of which arepivotable about generally horizontal pivot axes. In the embodiment shownin FIG. 1, support deck 30 includes a head section 42, a seat section44, a thigh section 46, and a foot section 48. Head section 42, which isalso sometimes referred to as a Fowler section, is pivotable about agenerally horizontal pivot axis between a generally horizontalorientation (not shown in FIG. 1) and a plurality of raised positions(one of which is shown in FIG. 1). Thigh section 46 and foot section 48may also be pivotable about generally horizontal pivot axes.

FIG. 2 illustrates in greater detail litter frame 28 separated fromlifts 26 and base 22. Litter frame 28 is also shown in FIG. 2 withsupport deck 30 removed. Litter frame 28 is supported by two lift headerassemblies 50. A first one of the lift header assemblies 50 is coupledto a top of a first one of the lifts 26, and a second one of the liftheader assemblies 50 is coupled to the top of the second one of thelifts 26. Each lift header assembly 50 includes a pair of load cells 52.The illustrated embodiment of person support apparatus 20 thereforeincludes a total of four load cells 52, although it will be understoodby those skilled in the art that different numbers of load cells may beused in accordance with the principles of the present disclosure. Loadcells 52 are configured to support litter frame 28. More specifically,load cells 52 are configured such that they provide complete andexclusive mechanical support for litter frame 28 and all of thecomponents that are supported on litter frame 28 (e.g. support deck 30,headboard 32, footboard 34, siderails 36, etc.). Because of thisconstruction, load cells 52 are adapted to detect the weight of not onlythose components of person support apparatus 20 that are supported bylitter frame 28 (including litter frame 28 itself), but also any objectsor persons who are wholly or partially being supported by support deck30.

The mechanical construction of person support apparatus 20, as shown inFIGS. 1 and 2, is the same as, or nearly the same as, the mechanicalconstruction of the Model 3002 S3 bed manufactured and sold by StrykerCorporation of Kalamazoo, Mich. This mechanical construction isdescribed in greater detail in the Stryker Maintenance Manual for theMedSurg Bed, Model 3002 S3, published in 2010 by Stryker Corporation ofKalamazoo, Mich., the complete disclosure of which is incorporatedherein by reference. It will be understood by those skilled in the artthat person support apparatus 20 can be designed with other types ofmechanical constructions, such as, but not limited to, those describedin commonly assigned, U.S. Pat. No. 7,690,059 issued to Lemire et al.,and entitled HOSPITAL BED; and/or commonly assigned U.S. Pat.publication No. 2007/0163045 filed by Becker et al. and entitled PATIENTHANDLING DEVICE INCLUDING LOCAL STATUS INDICATION, ONE-TOUCH FOWLERANGLE ADJUSTMENT, AND POWER-ON ALARM CONFIGURATION, the completedisclosures of both of which are also hereby incorporated herein byreference. The mechanical construction of person support apparatus 20may also take on forms different from what is disclosed in theaforementioned references.

Load cells 52 are part of a load cell system 54 (FIG. 3). Load cellsystem 54 includes, in addition to load cells 52, a controller 56, adetection circuit 58, a user interface 60, and an off-boardcommunication module 62. Load cell system 54 functions as a scale systemand/or as an exit detection system. When functioning as a scale system,load cell system 54 is adapted to measure the amount of weight that issupported on litter frame 28. Through the use of a tare control on userinterface 60, the weight of the litter frame 28 and other components ofthe person support apparatus 20 can be separated from the weight readingsuch that a weight of just the occupant of person support apparatus 20can be determined.

When load cell system 54 functions as an exit detection system, loadcell system 54 is adapted to determine when an occupant of personsupport apparatus 20 has left, or is likely to leave, person supportapparatus 20, and to issue an alert and/or notification to appropriatepersonnel so that proper steps can be taken in response to theoccupant's departure (or imminent departure) in a timely fashion. In atleast one embodiment, load cell system 54 acts as an exit detectionsystem by monitoring the center of gravity of the patient using thesystem and method disclosed in commonly assigned U.S. Pat. No. 5,276,432issued to Travis and entitled PATIENT EXIT DETECTION MECHANISM FORHOSPITAL BED, the complete disclosure of which is incorporated herein byreference. Other manners for functioning as an exit detection system arealso possible. Further, in some embodiments, load cell system 54functions both as an exit detection system and as a scale system.

Controller 56 is in communication with detection circuit 58 and eachload cell 52 (FIG. 3). Controller 56 is adapted to read the outputs fromeach load cell 52 and determine, based on the combination of outputs,the total weight or load that is being supported on litter frame 28. Inaddition, controller 56 is adapted to maintain and utilize a tare weightso that the weight of the occupant of person support apparatus 20 can bedistinguished from the weight of the components of person supportapparatus 20 and other non-patient items (e.g. bedding, pillows, etc.).In at least one embodiment, controller 56 is a microcontroller. It willbe understood, however, the controller 56 may take on other forms. Ingeneral, controller 56 may include any one or more microprocessors,microcontrollers, field programmable gate arrays, systems on a chip,volatile or nonvolatile memory, discrete circuitry, and/or otherhardware, software, or firmware that is capable of carrying out thefunctions described herein, as would be known to one of ordinary skillin the art. Such components can be physically configured in any suitablemanner, such as by mounting them to one or more circuit boards, orarranging them in other manners, whether combined into a single unit ordistributed across multiple units. The instructions followed bycontroller 56 in carrying out the functions described herein, as well asthe data necessary for carrying out these functions, are stored inmemory (not labeled) accessible to controller 56.

Controller 56, in addition to monitoring the outputs of load cells 52,may also may control other aspects of person support apparatus 20 (e.g.motion), or controller 56 may be in communication with one or more othercontrollers that control the other aspects of person support apparatus20.

Off-board communications module 62 includes one or more transceiversthat communicate with one or more off-board devices. In one embodiment,module 62 includes a WiFi radio adapted to communicate with wirelessaccess points of a healthcare facility's computer network, therebyenabling the person support apparatus 20 to communicate wirelessly withthe computer network of the healthcare facility. Module 62 may alsoinclude an Ethernet connection, or other wired circuitry, for enablingwired communication with the hospital network, as well as nurse callcable circuitry for coupling to a nurse call cable that communicateswith a nurse call system.

Detection circuit 58 is adapted to supply a substantially constantactivation voltage to load cells 52. Detection circuit 58 is alsoadapted to perform one or both of the following two additionalfunctions: (1) detecting whether one or more of the load cells 52 are inan error state (e.g. they are not present, are not electrically coupledto load cell system 54 properly, and/or are malfunctioning); and (2)detecting whether there are problems with the activation voltagesupplied to load cells 52. In carrying out either or both of thesefunctions, detection circuit 58 notifies controller 56 if it hasdetected an error with load cells 52 and/or an error with respect to theactivation voltage supplied to load cells 52. Controller 56, inresponse, sends a message to user interface 60 and/or off-boardcommunication module 62 indicating that an error has been detected. Userinterface 60 and/or the remote device in communication with module 62then alerts appropriate personnel in an audio, visual, and/oraudiovisual manner.

Each load cell 52 includes a pair of activation leads 64 and a pair ofsensing or sensor leads 66 (FIG. 3). Detection circuit 58 is inelectrical communication with activation leads 64, but not with sensorleads 66. Controller 56, in contrast, is in electrical communicationwith sensor leads 66, but not activation leads 64. One of each pair ofthe activation leads 64 of each load cell 52 is coupled to an activationvoltage source 68 (FIG. 4), which supplies electrical power to the loadcell 52, and the other of each pair of the activation leads 64 iscoupled to ground. Sensor leads 66, rather than supplying electricalpower, provide outputs to controller 56 that are used to determine howmuch force is being exerted on load cells 52. That is, sensor leads 66provide outputs that are correlated to the forces sensed by load cells52. In summary, activation leads 64 provide power for the load cells 52while sensors leads 66 provide outputs that are indicative of the forceapplied against load cells 52.

Although not shown in FIG. 3, load cells 52 are typically configured asWheatstone bridges wherein the one or more strain gauges that areinternal to the load cell are arranged in one or more legs of theWheatstone bridge. The other legs consist of known resistances. In otherwords, as shown in FIGS. 4-9, one of the strain gauges is effectivelyresponsible for one of the resistance values 76 a-d. (In some loadcells, strain gauges are positioned in more than one leg of theWheatstone bridge, but the strain gauges are geometrically arrangedwithin the load cell to sense the same magnitude, but not necessarilydirection, of the applied force). Typically, when no forces are detectedby the strain gauges of the load cell 52, the current flowing througheach of the two paths of the Wheatstone bridge is balanced, and there issubstantially no voltage drop between the two midpoints. However, when aforce is detected, the current is no longer balanced, and a voltage isdetected between the two midpoints. The two midpoints correspond to thesensor leads 66 while the two activation leads 64 correspond to theendpoints of the Wheatstone bridge. This can be seen more clearly inFIGS. 4-9, which show the internal Wheatstone bridge configuration ofthe load cells 52 and the locations of activation and sensing leads 64and 66.

FIGS. 4-9 illustrate six different manners in which detection circuit 58can be used to detect an error state with respect to one or more loadcells 52 and/or an error state with the activation voltage supplied tothe load cells 52. These six different manners of implementing detectioncircuit 58 will be referenced as detection circuits 58 a-f,respectively. In some of these embodiments, detection circuit 58 onlychecks for error states in the load cells 52, while in others of theseembodiments, detection circuit 58 checks for both error states with theload cells and with the activation voltage source, as will now bediscussed in greater detail below.

Detection circuit 58 a of FIG. 4 is adapted to detect the presence orabsence of each load cell, as well as the status of each load cell 52that is present. A load cell is considered “absent” if it is physicallynot part of person support apparatus 20, if it is attached to personsupport apparatus 20 but not electrically integrated with detectioncircuit 58 a and/or controller 56, or if it is attached to personsupport apparatus 20 and electrically integrated, but not electricallyintegrated in a complete or proper manner. Additionally, not only isdetection circuit 58 a adapted to detect if there is an error state withrespect to a load cell (e.g. the load cell is absent or malfunctioning),but it is also adapted to identify which specific one (or ones) of theload cell is in the error state, and to report that identification tocontroller 56.

Detection circuit 58 a includes an activation voltage source 68, a senseresistor 70, and four switches 72 a-d. Sense resistor is used to measurethe amount of electrical current that is flowing out of activationvoltage source 68 and to load cells 52. That is, circuitry (not shown)is coupled to sense resistor 70 that measures the amount of currentflowing through it and reports this measured current amount tocontroller 56. As can be seen in FIG. 4, sense resistor is arranged inseries with each of the load cells 52.

Each switch 72 a-d is an electronically controlled switch (such as, forexample, a Metal Oxide Semiconducting Field Effect Transistor: MOSFET),that is opened and closed by controller 56. As shown in FIG. 4, eachswitch 72 a-d is arranged in series with each other such that, when allof the switches 72 a-d are closed, electrical current from activationvoltage source 68 is supplied to all of the load cells 52. In contrast,when all of the switches 72 a-d are open, no electrical current flows toany of the load cells 52, nor through sense resistor 70. If switch 72 ais closed, but the three other switches are open, electrical current issupplied to only the first load cell 52 a, but not to the other loadcells 52 b-d.

During normal operation of the load cell system, all four of theswitches 72 a-d are closed so that electrical current is supplied to allfour load cells 52 a-d, thereby allowing force readings to be read fromeach load cell 52 a-d (via sensor leads 66). Also during normaloperation, the amount of current flowing through sense resistor 70 ismonitored by controller 56. As will be discussed in greater detailbelow, if the amount of current flowing through sense resistor 70 isoutside of a predefined range, this is indicative of an error state withrespect to one or more load cells 52 a-d and/or with respect toactivation voltage source 68. If controller 56 determines that thecurrent through sense resistor 70 is outside of this range, controller56 selectively opens and closes switches 72 a-d of detection circuit 58a in order to determine which specific one (or ones) of load cells 52a-d are in the error state, as will also be discussed more below.

Before turning to the detailed manner in which detection circuit 58 a isused to determine error states of load cells 52 and/or activationvoltage source 68, it will be helpful to explain the operation of theload cell system when no error states exist. During normal operation(e.g. no error states), the amount of electrical current that flowsthrough each load cell 52 (and also through sense resistor 70) issubstantially constant. As will be discussed, some minor variationsoccur in response to different amounts of weights that are detected bythe load cell system, but these variations are orders of magnitudesmaller than the current variations that normally occur when a load cell52 is absent, unplugged, malfunctioning, or in some other type of errorstate. Controller 56 therefore ignores these minor changes in thecurrent through sense resistor 70 that are due to changing forcesapplied to the load cells, and instead monitors whether or not moresubstantial changes in the current through sense resistor 70 occur. Onlyif such substantial current changes occur does controller 56 determinethat an error state occurs.

The relatively constant amount current that is expected to flow throughsense resistor 70 during normal operation is calculated during thedesign of person support apparatus 20. This value (herein “the normalcurrent value”) is calculated based upon the voltage of activationvoltage source 68, the size of sense resistor 70, and the amount ofelectrical resistance each load cell 52 has (which depends upon theparticular load cells 52 that are selected for use in person supportapparatus 20). Inside of each load cell 52 is a Wheatstone bridge 74 (asdiscussed above) that includes four resistance values 76 a-d. In atypical configuration, the value of three of those resistance values 76a-d is known, while the value of the fourth one is variable based uponthe amount of strain applied to the load cell. In other words, the oneor more strain gauges inside of the load cell 52 are typically arrangedto occupy the leg of the Wheatstone bridge 74 having an unknown andvariable resistance value. When no force is applied to the straingauges, the resistances from the other three resistors 76 are configuredsuch that the Wheatstone bridge is in balance and there is substantiallyno voltage difference between activation leads 64. (In some cases, avoltage may be present, but this can be accounted for via calibration,as will be discussed more below).

When a force is applied to the load cell, the value of the resistance inone leg of the Wheatstone bridge 74 changes in an amount that iscorrelated in a known manner to the amount of the applied force. Thischange in the resistance causes a change in the voltage drop between theactivation leads 64, which will, in turn, change the amount of currentflowing from a first one of the activation leads 64 to the second one ofthe activation leads 64. Further, the change in current between the twoactivation leads 64 of an individual load cell 52 causes a change in thecurrent flowing through sense resistor 70. However, as mentioned above,this change in electrical current will be relatively small. In someembodiments, the collective difference in current through the activationleads 64 of all four load cells 52 when they have no force applied tothem and when they have their maximum force applied to them is on theorder of between 1-10 microamperes. In contrast, in some embodiments,when one or more of the load cells 52 is in an error state, the currentflowing through sense resistor 70 to the load cells 52 will change in anamount ranging from 5-30 milliamperes. (It will be understood that theranges of these microamp and milliamp changes are merely illustrative ofsome embodiments, and that other changes are possible in differentembodiments). Regardless of the specific values, however, the change incurrent due to an error state will be at least an order of magnitudedifferent from the change in current due to changing loads applied tothe load cells.

Controller 56 is therefore programmed with the normal current value thatis expected to flow through resistor 70 when there are no error states,and periodically compares readings of the actual current flowing throughresistor 70 with the normal current value. If there is a deviation thatis sufficiently large to indicate that a load cell 52 is in an errorstate, controller 56 proceeds to identify which of the four load cellsis in the error state. This identification process is accomplishedthrough the use of switches 72 a-d.

More specifically, when controller 56 determines that one or more of theload cells 52 are in an error state due to the current flowing throughresistor 70 having changed by greater than a threshold amount,controller 56 keeps switch 72 a closed while opening switches 72 b-d. Inthis configuration of switches 72, current through resistor 70 is onlyallowed to flow through load cell 52 a. Controller 56 takes ameasurement of the current flowing through resistor 70 in thisconfiguration and compares it to a first stored expected current valuethat corresponds to the expected amount of current to flow through loadcell 52 a alone. This first expected current value is stored in a memoryaccessible to controller 56 and is determined during the design of theload cell system. If the comparison shows that the current through senseresistor 70 is within an acceptable range of the first stored expectedcurrent value, then controller 56 concludes that the first load cell 52a is functioning properly. If the comparison shows that the currentthrough sense resistor 70 is outside of the acceptable range of thefirst stored expected current value, then controller 56 concludes thatload cell 52 a is not functioning properly and stores that informationin memory, along with the value of the current flowing through firstload cell 52 a.

In at least one embodiment, controller 56 moves onto testing the secondload cell 52 b regardless of whether or not first load cell 52 a was inan error state or not. That is, even if first load cell 52 a wasdetermined to be malfunctioning, controller 56 is configured in at leastone embodiment to also test the other load cells 52 b-d before sendingan error message to user interface 60 and/or communications module 62.In order to test second load cell 52 b, controller 56 closes secondswitch 72 b while keeping first switch 72 a closed (and third and fourthswitches 72 c and 72 c open). This allows current from activationvoltage source 68 to flow through both first and second load cells 52 aand 52 b (as well as through sense resistor 70). Controller 56 detectsthe amount of current flowing through resistor 70 and compares thiscurrent value with a second stored expected current value.

The second stored expected current value corresponds to an expectedamount of current to flow through load cells 52 a and 52 b whenactivation source voltage 68 and load cells 52 a and 52 b are allfunctioning correctly. As with the first stored expected current value,this second expected current value is stored in a memory accessible tocontroller 56 and is determined during the design of the load cellsystem. If the comparison shows that the current through sense resistor70 is within an acceptable range of the second stored expected currentvalue, then controller 56 concludes that the second load cell 52 b isfunctioning properly. If the comparison shows that the current throughsense resistor 70 is outside of the acceptable range of the secondstored expected current value, then controller 56 concludes that loadcell 52 b is not functioning properly and stores that information inmemory, along with the value of the total current flowing through senseresistor 70 (which then flows through first and second load cells 52 aand 52 b).

As mentioned above, if controller 56 determines that the current flowingthrough first load cell 52 a was outside of an acceptable range of thefirst stored expected current value, controller 56 stores the value ofthis incorrect current flow in memory. Further, controller 56 uses thisincorrect current flow to adjust the second expected current flow whendetermining whether or not the second load cell 52 b is in an errorstate or not. That is, in at least one embodiment, controller 56calculates the resistance through first load cell 52 a based upon theincorrect current flow through first load cells 52 a (and sense resistor70), uses this calculated resistance to determine the combinedresistance of first and second load cells 52 a and 52 b when switch 72 bis closed (using the known and stored expected resistance of second loadcell 52 b), and then, using this calculated resistance, determines ifthe current through sense resistor 70 matches what would be expected toflow therethrough when switches 72 a and 72 b are closed (and the restare open). In this manner, even though first load cell 52 a wasdetermined to be in an error state, controller 56 is able to account forthis error state to determine whether or not second load cell 52 b isalso in an error state.

Controller 56 continues in a similar manner to check the error states ofload cells 52 c and 52 d by subsequently closing third switch 72 c andcomparing the current through sense resistor 70 to a third expectedcurrent value, and then closing the fourth switch 72 d and comparing thecurrent through sense resistor 70 to a fourth expected current value.Controller 56 is thereby able to not only detect that one or more of theload cells 52 are in an error state, but also to identify which ones ofthe load cells 52 are in an error state. After identifying the one ormore load cells that are in an error state, controller 56 forwards thisinformation to user interface 60 and/or off-board communications module62 so that appropriate personnel are notified of the error state.

In an alternative embodiment, detection circuit 58 a is modified so thateach of switches 72 a-d are arranged in parallel with each other and inseries with a corresponding one of load cells 52 (e.g. switch 72 a is inseries with load cell 52 a, switch 72 b is in series with load cell 52b, etc.). In this alternative embodiment, controller 56 identifies whichone(s) of the load cells are in an error state by closing thecorresponding switch of only the one load cell that is being tested(e.g. closing switch 72 c to test load cell 52 c, while keeping theremaining switches 72 a-b and d open). This allows controller 56 to moreeasily identify if multiple load cells are in an error state because thecurrent through sense resistor 70 due to the error state of one loadcell 52 can be isolated from the current flowing through sense resistor70 when another load cell is tested. This isolation is achieved byopening the switch corresponding to the load cell that is in the errorstate while the other load cell is tested, thereby removing the loadcell in the error state from the current flow.

FIG. 5 illustrates an alternative detection circuit 58 b. Detectioncircuit 58 b of FIG. 5, like detection circuit 58 a of FIG. 4, isadapted to detect the presence or absence of each load cell, as well asthe status of each load cell 52 that is present. Further, detectioncircuit 58 b is adapted to detect if activation voltage source 68 is inan error state. Those components of detection circuit 58 b that arecommon to detection circuit 58 a are numbered in FIG. 5 with the samenumbers as in FIG. 4, and operate in the same manner as discussed above.Those components of detection circuit 58 b that are not found indetection circuit 58 a are provided with a new reference number anddescribed in more detail below.

Detection circuit 58 b differs from detection circuit 58 a in that itincludes a fifth switch 72 e. Fifth switch 72 e is operated under thecontrol of controller 56 and is used to determine whether or notactivation voltage source 68 is in an error state or not. Controllerdetermines whether or not activation voltage source 68 is in an errorstate or not by opening switch 72 a and closing switch 72 e. In thisconfiguration, current from activation voltage source 68 is only able toflow through sense resistor 70 and switch 72 e. Because the value ofsense resistor 70 is known to controller 56, and because activationvoltage source 68 is designed to supply a substantially constant andknown voltage, the expected value of the current flowing through senseresistor 70 is also known. More specifically, the expected value of thecurrent flowing through sense resistor 70 when switch 72 a is open andswitch 72 e is closed is stored in a memory accessible to controller 56.Controller 56 then reads the actual amount of current flowing throughsense resistor 70 when switch 72 a is open and switch 72 e is closed andcompares it to this stored expected value. If the value differs by morethan a threshold, controller 56 determines that activation voltagesource 68 is not supplying the expected constant voltage and istherefore in an error state. Controller 56 sends a message to userinterface 60 and/or off-board communications module 62 that indicatesthat activation voltage source 68 is in an error state so thatnotification can be provided to appropriate personnel.

Controller 56 closes switch 72 e in detection circuit 58 b at selectedtimes, but not during normal operation of the load cell system. That is,controller 56 does not close switch 72 e when the outputs of load cells52 a-d (e.g. the voltages across sensor leads 66) are being monitored todetermine whether forces are being applied to load cells 52 a-d.Instead, controller 56 closes switch 72 e and checks whether activationvoltage source 68 is in an error state when the load cell system isinitially powered, or at times when the load cells system is not beingused (e.g. when the exit detection function is not armed and/or when ascale reading is not being taken). When the load cell system is beingused to detect forces applied to the load cells 52 a-d, controller 56keeps switch 72 e open and monitors the current through sense resistor70 in the manner described above to determine if any of the load cells52 a-d are in an error state.

FIG. 6 illustrates another alternative detection circuit 58 c. Detectioncircuit 58 c of FIG. 6, like detection circuits 58 a and 58 b of FIGS. 4and 5, is adapted to detect the presence or absence of each load cell,as well as the status of each load cell 52 that is present. Further,like detection circuit 58 b, detection circuit 58 c is adapted to detectif activation voltage source 68 is in an error state. Those componentsof detection circuit 58 c that are common to detection circuits 58 a or58 b are numbered in FIG. 6 with the same numbers as used with detectioncircuit 58 a and/or 58 b, and operate in the same manner as discussedabove. Those components of detection circuit 58 c that are not found indetection circuits 58 a and/or 58 b are provided with a new referencenumber and described in more detail below.

Detection circuit 58 c differs from detection circuit 58 b in that itincludes a second resistor 78 instead of fifth switch 72 e. Secondresistor 78 is in parallel with each of the load cells 52 and in serieswith respect to the sense resistor 70. Controller 56 monitors thecurrent flowing through sense resistor 70 in the same manner asdescribed above with respect to detection circuit 58 a. That is,controller 56 monitors the current flowing through sense resistor 70 atall times when the load cell system is in use, and compares the currentflow through resistor 70 with an expected current flow. If the measuredcurrent flow deviates from the expected current flow by more than anacceptable tolerance, then controller 56 determines that there is anerror state. However, unlike detection circuit 58 a, which is notadapted to distinguish between an error state due to activation voltagesource 68 and one due to one or more load cells 52, detection circuit 58c allows controller 56 to make this determination. This is accomplishedas follows.

When controller 56 determines that an unexpected amount of current isflowing through sense resistor 70, it opens switch 72 a and measures thecurrent flowing through sense resistor 70 when switch 72 a is opened.Controller 56 has access to a memory having stored therein an expectedvalue of current to flow through sense resistor 70 when switch 72 a isopened. This expected value is calculated during the design of the loadcell system and is based upon the known expected value of activationvoltage source 68 and the known resistance values of sense resistor 70and second resistor 78. If the current flowing through sense resistor 70when switch 72 a is open deviates from the expected value by more thanan acceptable tolerance, controller 56 determines that activationvoltage source 68 is not supplying the expected constant voltage and istherefore in an error state. If the current flowing through senseresistor 70 when switch 72 a is open does not deviate from the expectedvalue by more than the tolerance, controller 56 determines that theprevious unexpected current flow through sense resistor 70 while switch72 a was closed was due to an error in one or more load cells 52 a-d,not due to activation voltage source 68. Controller 56 therefore thenproceeds to close switch 72 a and cycle through the steps describedabove to identify which of the four load cells 52 a-d is in the errorstate.

FIG. 7 illustrates another alternative detection circuit 58 d. Detectioncircuit 58 d operates substantially the same as detection circuit 58 aof FIG. 4. That is, detection circuit 58 d of FIG. 7, like detectioncircuit 58 a of FIG. 4, is adapted to detect the presence or absence ofeach load cell, as well as the status of each load cell 52 that ispresent. However, unlike detection circuit 58 a, detection circuit 58 d,does not include switches 72 b, 72 c, and 72 d. Detection circuit 58 dtherefore does not allow controller 56 to automatically identify whichspecific load cell 52 is in an error state. Instead, controller 56 isadapted to issue an alert when the current through sense resistor 70deviates from an expected value by more than a threshold, and leave itup to appropriate personnel to determine which specific one (or ones) ofthe load cells 52 a-d is in an error state.

In some embodiments, detection circuit 58 d may be further modified byremoving switch 72 a altogether. When so modified, controller 56monitors the current through sense resistor 70 and issues an alert ifthe monitored current deviates by more than the tolerance from theexpected value.

FIG. 8 illustrates another alternative detection circuit 58 e. Detectioncircuit 58 e operates substantially the same as detection circuit 58 bof FIG. 5. That is, detection circuit 58 e of FIG. 8, like detectioncircuit 58 b of FIG. 5, is adapted to detect the presence or absence ofeach load cell, the status of each load cell 52 that is present, andwhether activation voltage source 68 is in an error state or not.However, unlike detection circuit 58 b, detection circuit 58 e does notinclude switches 72 b, 72 c, and 72 d. Detection circuit 58 e thereforedoes not allow controller 56 to automatically identify which specificload cell 52 is in an error state. Instead, controller 56 is adapted toissue an alert when the current through sense resistor 70 deviates froman expected value by more than a threshold, and leave it up toappropriate personnel to determine which specific one (or ones) of theload cells 52 a-d is in an error state. Controller 56 also issues analert if it detects an error state with respect to activation voltagesource 68 (which, as with detection circuit 58 b, is detected by closingswitch 72 e and opening switch 72 a).

FIG. 9 illustrates another alternative detection circuit 58 f. Detectioncircuit 58 f operates substantially the same as detection circuit 58 cof FIG. 6. That is, detection circuit 58 f of FIG. 9, like detectioncircuit 58 c of FIG. 6, is adapted to detect the presence or absence ofeach load cell, the status of each load cell 52 that is present, andwhether activation voltage source 68 is in an error state or not.However, unlike detection circuit 58 c, detection circuit 58 f does notinclude switches 72 b, 72 c, and 72 d. Detection circuit 58 f thereforedoes not allow controller 56 to automatically identify which specificload cell 52 is in an error state. Instead, controller 56 is adapted toissue an alert when the current through sense resistor 70 deviates froman expected value by more than a threshold, and leave it up toappropriate personnel to determine which specific one (or ones) of theload cells 52 a-d is in an error state. Controller 56 also issues analert if it detects an error state with respect to activation voltagesource 68 (which, as with detection circuit 58 c, is detected by openingswitch 72 a).

In all of the detection circuits above, it will be understood thatswitches 72 a-e can be implemented in a variety of different manners. Asnoted, in one embodiment, switches 72 a-e are MOSFETs that arecontrolled by controller 56. In other embodiments, switches 72 a-e areimplemented using other types of transistors, or implemented usingrelays. Still other types of switching may be implemented. In stillother embodiments, one or a combination of switches 72 b, 72 c and 72 dis added or removed from the circuit to achieve similar function asmentioned above.

FIG. 10 illustrates another alternative load cell system 54 a. Load cellsystem 54 a may be used with any of the person support apparatusesdescribed herein. Those components of load cell system 54 a that arecommon to load cell system 54 are numbered with the same referencenumbers. Those components of load cell system 54 a that are not found inload cell system 54, or that are modified from load cell system 54, areprovided with a new or modified reference number and described in moredetail below. For purposes of clarity, the connections betweencontroller 56 and each of the sensor leads 66 have not been shown.However, it will be understood that such electrical connections existand are used by controller 56 in the same manners previously described.

Load cell system 54 a differs from load cell system 54 in that itincludes a detection circuit 58 g that is applied to load cells that arearranged in a Wheatstone bridge configuration, rather than in parallelto each other (as with detection circuits 58 a-f). Detection circuit 58g operates to detect the presence or absence of each load cell, thestatus of each load cell 52 that is present, and whether activationvoltage source 68 is in an error state or not. Although not visible inFIG. 10, detection circuit 58 g includes an internal sense resistor 70and a substantially constant activation voltage source 68. Detectioncircuit 58 g operates in conjunction with four load cells 52 a-d thatare collectively arranged as a Wheatstone bridge 74 e. That is, loadcell 52 a is positioned in a first leg of bridge 74 e; load cell 52 b ispositioned in a second leg of bridge 74 e, third load cell 52 c ispositioned in a third leg of bridge 74 e; and fourth load cell 52 d ispositioned in a fourth leg of bridge 74 e. The activation voltage sourceof detection circuit 58 g supplies a substantially constant voltage tofirst activation leads 64 of load cells 52 a and 52 b. The secondactivation leads 64 of first and second load cells 52 a and 52 b arecoupled to the first activation leads 64 of third and fourth load cells52 c and 52 d. The second activation leads 64 of third and fourth loadcells 52 c and 52 d are coupled to ground (and/or back to detectioncircuit 58 g).

Detection circuit 58 g monitors the amount of current flowing throughits sense resistor 70 during normal operation of the load cells 52 a-d.This measured amount of current should be substantially constant,although it will have minor variations caused by the differing forcesapplied to the load cells 52. When detection circuit 58 g determinesthat the current through the sense resistor has deviated from theexpected current by more than an acceptable tolerance, it concludes thatone of the four load cells is in an error state.

Detection circuit 58 g also detects the voltage between midpoints 80 aand 80 b of the Wheatstone bridge 74 e and forwards this voltagemeasurement to controller 56. In at least one embodiment, controller 56compares the voltage between midpoints 80 a and 80 b when no load isbeing applied to the load cells 52 and a stored, expected no-loadvoltage value. The stored, expected no-load value is determined duringthe design and/or manufacturing of the person support apparatus 20 bymeasuring the voltage across midpoints 80 a and 80 b when no load ispresent and all of the load cells 52 a-d are known to be workingproperly. In at least one embodiment, this stored, expected no-loadvoltage value is substantially zero volts. If the measured voltagebetween midpoints 80 a and 80 b deviates from this stored, expectedno-load voltage by more than an acceptable tolerance, then controller 56concludes that one or more of the load cells 52 a-d, or the activationvoltage source 68, in is an error state. In at least one embodiment, ifcontroller 56 determines that the current flowing through the senseresistor 70 is within the acceptable tolerance of its expected value,but that the voltage between midpoints 80 a and 80 b during a no-loadsituation deviates from its expected value by more than an acceptablethreshold, then controller 56 concludes that activation voltage source68 is in an error state.

The arrangement of load cells 52 a-d in a Wheatstone bridgeconfiguration therefore enables controller 56 to determine whether ornot activation voltage source 68 is in an error state without the use ofany switches. This is accomplished in one embodiment by designing theload cell system such that load cells 52 a-d, during a no-load, no-errorstate configuration, have resistances that are balanced in theWheatstone bridge 74 e. When so balanced, substantially no voltageexists between midpoints 80 a and 80 b. Therefore, if controller 56detects a voltage between midpoints 80 a and 80 b during a no-loadmoment, controller 56 concludes that an error state exists in one ormore of the load cells 52 and/or with activation voltage source 68.Further, as noted above, if controller 56 detects that the currentthrough the sense resistor 70 deviates from its expected value by morethan an acceptable threshold, then controller 56 concludes that theerror state is due to activation source voltage 68, rather than one ormore individual load cells 52.

Although the foregoing operation has been primarily described withrespect to a no-load situation, it will be understood that it may applyequally to any load situation where the applied load value is known andmeasurements of the voltage between points 80 a and 80 b during thatknown load were taken and recorded at a time when the load cell systemwas functioning properly. In such situations, controller 56 compares thevoltage detected between midpoints 80 a and 80 b with the previouslyrecorded measurements taken when no error state existed. If they differby more than a threshold, controller 56 determines that an error stateexists.

In at least one embodiment, user interface 60 is adapted to allow asuitable person, such as a technician, to input information tocontroller 56 that indicates that no load is being applied to the loadcells 52. When such information is input, controller 56 takesmeasurements of the voltage between midpoints 80 a and 80 b and comparesthem to their expected value to see if an error state exists. Userinterface 60 may also be adapted to allow the technician, or otherauthorized personnel, to input information that identifies the specificvalue of a load being applied to load cells 52. For example, thetechnician may place one or more known weights on the load cells systemand inform controller 56 of the amount of this weight via user interface60. Controller 56 then compares the measured voltage between midpoints80 a and 80 b with the expected voltage for the input weight and issuesan error message if there is a deviation.

FIG. 11 depicts an another alternative load cell system 54 b that may beused with any of the person support apparatuses described herein. Thosecomponents of load cell system 54 b that are common to load cell systems54 and/or 54 a are numbered with the same reference numbers. Thosecomponents of load cell system 54 b that are not found in load cellsystems 54 or 54 a, or that are modified from load cell systems 54 or 54a, are provided with a new or modified reference number and described inmore detail below.

Load cell system 54 b differs from load cell systems 54 and 54 a in thatit includes load cells 52 e and 52 f that are configured differentlyfrom load cells 52 a-d. More specifically, load cells 52 e and 52 f areconfigured to include an internal load cell memory 82. Load cell memory82 includes calibration data stored therein that is specific to eachload cell 52 e and 52 f. Such calibration data includes one or more ofthe following items: a Y value (E_(MAX)/V_(MIN)), rate output (e.g.mV/V), creep (over a given time period), temperature effect on output,temperature effect on zero, input impedance, output impedance, etc. Allof the calibration values that are stored in memory 82 are specific to aparticular load cell, and may be different for different load cells.Memory 82 is adapted to be read by an external controller that is notpart of the load cells 52 e or 52 f, such as, controller 56.

Controller 56 uses the stored calibration values within a particularload cell memory 82 to convert the voltage outputs from the sensor leads66 of that particular load cell into an accurate weight value. Byincluding this calibration data within load cell memory 82, it is notnecessary for a user to manually program this information into the codeexecuted by controller 56, or otherwise manually enter this information.Instead, controller 56 can read this information automatically and inputit into its weight determining algorithm without requiring further humaninput. Including this calibration data within the load cell memory 82also helps avoid mistakes being made during the manufacturing orconfiguration of person support apparatus 20 wherein calibration datafor a particular load cell 52 is entered for the wrong load cell. Inother words, mismatches between a load cell and its specific calibrationdata are avoided by including this calibration data in a memory that isphysically integrated into the load cells 52 e and/or 52 f.

The load cell calibration data contained with load cell memories 82 isloaded into memories 82 either by the manufacturer of the load cell 52,or by a testing entity (e.g. an agency, contractor, laboratory, etc.)that performs testing on the load cells after the load cells have beenmanufactured. In some embodiments, the load cells 52 are manufacturedwithout a memory, and both the memory and the calibration data are addedto the load cell 52 after the calibration data is gathered throughsufficient testing. The manner in which the memory 82 is secured to theload cell may vary so long as it is difficult for the memory to becomephysically disassociated from the corresponding load cell.

As shown in FIG. 11, load cell system 54 b also includes a personsupport apparatus memory 84 that is positioned on board person supportapparatus 20 and in communication with controller 56. Person supportapparatus memory 84 is separate from load cell memories 82. That is,person support apparatus memory 84 remains with person support apparatus20 when load cells 52 e and/or 52 f are removed or replaced. Personsupport apparatus memory 84 stores calibration data for the load cells52 e and 52 f that is specific to the person support apparatus 20. Thatis, after load cells 52 e and 52 f are installed on person supportapparatus 20, known forces are applied to the load cell system and thevalues measured by each load cell 52 e and 52 f are recorded in personsupport apparatus memory 84. These values take into account the mannersin which the load cells 52 e and 52 f react to forces applied to themafter they have been integrated into a person support apparatus 20,which may be different from how they react to forces applied on a testbench, or in some other environment outside of a particular personsupport apparatus 20.

Controller 56 uses the load cell calibration data stored in memory 84,along with the calibration data stored in load cell memories 82, whenconverting the outputs of load cells 52 e and 52 f into forcemeasurements. Further, when a particular load cell 52 has to bereplaced, controller 56 is adapted, in at least one embodiment, to usethe calibration data from the load cell memory 82 of the replacementload cell 52 in combination with the calibration data from the personsupport apparatus memory 84 to process the outputs from the replacementload cell. Controller 56 does this without requiring additionalcalibration tests to be run on the person support apparatus 20 after thereplacement load cell has been installed. That is, once the calibrationdata that is specific to the person support apparatus 20 is determinedand stored in memory 84, controller 56 uses this patient supportapparatus-specific data with any future replacement load cells such thatthe process of applying known weights to the load cell system with thereplacement load cells and recording their outputs does not have to beperformed.

Although not illustrated in FIG. 11, it will be understood that loadcell system 54 b can be modified to include any one or the variousdetection circuits 58 a-g discussed above so that controller 56 candetect error states of the one or more load cells 52 e-f, and/or of theactivation voltage source. Further, it will be understood that, althoughload cell system 54 b is shown in FIG. 11 with only two load cells, itsfunctions and features can be applied to load cell systems havingdifferent numbers of load cells. Similarly, although load cell systems54 and 54 a have been illustrated herein as including four load cells,the functions and features of these systems can be applied to load cellsystems having different numbers of load cells.

In some embodiments of load cell system 54 b, controller 56 transfersthe data stored within each load cell memory 82 to the person supportapparatus memory 84. The person support apparatus memory 84 thencontains two subsets of calibration data for each load cell: a firstsubset that is specific to the individual load cell, and a second subsetthat is specific to the person support apparatus 20. As noted,controller 56 uses both of these subsets when converting the voltageoutputs from the load cells into weight or force readings.

Although load cells 52 e and 52 f are the only load cells shown anddescribed herein as containing load cell memories 82 having calibrationdata stored therein, it will be understood that any of load cells 52 a-dthat are used in load cell system 54 and 54 a could be modified toinclude load cell memories 82 with calibration data stored therein, andthat any of the detection circuits 58, 58 a-58 g can be used with loadcells having load cell memories 82 with calibration data stored therein.

It will also be understood by those skilled in the art that, althoughthe load cell systems discussed herein have primarily been describedwith respect to a scale and/or exit detection system of a person supportapparatus 20, the load cell systems described herein can also be appliedto load cells that are used on person support apparatuses 20 for otherpurposes. For example, any of the features of the load cell systemdescribed herein can alternatively be used with one or more load cellsthat are used in conjunction with a self-propulsion system that may bebuilt into the person support apparatus. Two examples of suchself-propulsion systems are disclosed in commonly assigned U.S. Pat. No.6,772,850, issued to Waters et al. and entitled POWER ASSISTED WHEELEDCARRIAGES, as well as U.S. patent publication 2014/0076644 publishedMar. 20, 2014 by inventors Richard Derenne et al. and entitled POWEREDPATIENT SUPPORT APPARATUS, the complete disclosures of both of which arehereby incorporated herein by reference.

It will also be understood by those skilled in the art that controller56 may be programmed, in any of the load cell system described herein,to also monitor the voltages detected between sensor leads 66 todetermine whether or not a load cell is in an error state. That is, if aload cell 52 generates a voltage between sensor leads 66 thatcorresponds to a weight value that is outside of the expected ordesigned range of acceptable weight values, then controller 56 isprogrammed to conclude that the load cell is in an error state. Theerror state detected by examining the voltage between sensor leads 66may not necessarily be detectable as an error state by examining thevoltage between activation leads 64, or vice versa. By programmingcontroller 56 to monitor the voltages across both the activation leads64 and the sensor leads 66, controller 56 may be able to more robustlydetect error states with one or more of the load cells 52 and/or theactivation voltage source 68.

Still further, it will be understood by those skilled in the art that,although FIG. 11 depicts load cells 52 e and 52 f as containing twostrain gauges each, the number of strain gauges within a particular loadcell 52 can vary. That is, the principles disclosed herein are alsoapplicable to load cells having only a single strain gauge and/or toload cells having more than two strain gauges.

Various additional alterations and changes beyond those alreadymentioned herein can be made to the above-described embodiments. Thisdisclosure is presented for illustrative purposes and should not beinterpreted as an exhaustive description of all embodiments or to limitthe scope of the claims to the specific elements illustrated ordescribed in connection with these embodiments. For example, and withoutlimitation, any individual element(s) of the described embodiments maybe replaced by alternative elements that provide substantially similarfunctionality or otherwise provide adequate operation. This includes,for example, presently known alternative elements, such as those thatmight be currently known to one skilled in the art, and alternativeelements that may be developed in the future, such as those that oneskilled in the art might, upon development, recognize as an alternative.Any reference to claim elements in the singular, for example, using thearticles “a,” “an,” “the” or “said,” is not to be construed as limitingthe element to the singular.

What is claimed is:
 1. A person support apparatus comprising: a frame; a plurality of load cells supported by the frame, wherein each load cell includes a first set of leads for powering the load cell and a second set of leads for outputting a signal that varies as a function of a physical load applied to the load cell; a support surface adapted to support thereon an occupant of the person support apparatus, the support surface being supported by the load cells such that a weight of the occupant is detectable by the load cells when the occupant is positioned on the support surface; a detection circuit in communication with the first set of leads but not the second set of leads of the load cells; and a controller in communication with the detection circuit, the controller adapted to determine if any of the plurality of load cells are in an error state based upon information from the detection circuit, the controller further adapted to output an error signal if one or more of the plurality of load cells are in the error state, wherein the error state comprises one or more of the plurality of load cells not being electrically coupled to an activation power source.
 2. The person support apparatus of claim 1 wherein the detection circuit detects changes in a total amount of electrical current supplied to all of the first sets of leads from the activation power source.
 3. The person support apparatus of claim 2 wherein the controller determines that one or more of the plurality of load cells are not electrically coupled to the activation power source if the total amount of electrical current supplied to all of the first sets of leads is outside of a predetermined range.
 4. The person support apparatus of claim 3 wherein the predetermined range is a function of the number of load cells in the plurality of load cells.
 5. The person support apparatus of claim 1 wherein the detection circuit detects changes in electrical current for each of the first sets of leads.
 6. The person support apparatus of claim 5 wherein the controller is adapted to identify individual ones of the plurality of load cells that are present.
 7. The person support apparatus of claim 6 wherein the controller identifies an individual one of the plurality of load cells as being present if an amount of electrical current supplied to the first set of leads to the individual one of the plurality of load cells falls within a predetermined range.
 8. The person support apparatus of claim 1 wherein the detection circuit includes a plurality of switches controlled by the controller, each one of the switches being arranged in series with a corresponding one of the plurality of load cells.
 9. The person support apparatus of claim 1 further comprising: a plurality of load cell memories, wherein each of the plurality of load cell memories is physically coupled to a respective one of the plurality of load cells, each load cell memory including first calibration data regarding the respective load cell stored therein, the first calibration data generated before the respective load cell is installed in the person support apparatus; a person support apparatus memory containing second calibration data regarding the plurality of load cells, the second calibration data generated after the plurality of load cells are installed in the person support apparatus; and wherein the controller is adapted to use the first calibration data and the second calibration data to determine the weight of the occupant.
 10. The person support apparatus of claim 9 wherein the person support apparatus is one of a bed, a recliner, a cot, and a stretcher.
 11. The person support apparatus of claim 10 wherein the controller is further adapted to use the second calibration data to calibrate a replacement load cell when the replacement load cell is installed in the person support apparatus that replaces one of the plurality of load cells.
 12. The person support apparatus of claim 11 wherein the controller is adapted to not use the first calibration data when the replacement load cell is installed in the person support apparatus.
 13. The person support apparatus of claim 11 wherein the controller calibrates the replacement load cell without requiring any calibration measurements to be taken regarding the replacement load cell after the replacement load cell is installed in the person support apparatus.
 14. The person support apparatus of claim 1 wherein a first one of the plurality of load cells is included within a first Wheatstone bridge, a second one of the plurality of load cells is included within a second Wheatstone bridge, the first and second Wheatstone bridges are included within a third Wheatstone bridge, and the controller monitors voltages between midpoints of each of the first, second, and third Wheatstone bridges.
 15. The person support apparatus of claim 14 wherein the controller uses the voltage between the midpoints of the first Wheatstone bridge and the voltage between the midpoints of the second Wheatstone bridge when determining the weight of the occupant.
 16. The person support apparatus of claim 15 wherein the controller uses the voltage between the midpoints of the third Wheatstone bridge to determine if a second error state exists for at least one of the first and second load cells when no weight is being exerted on the support surface by the occupant, the second error state being different from the error state.
 17. The person support apparatus of claim 15 further including a detection circuit in communication with the first and second load cells and the controller, wherein the controller is further adapted to determine if an error state exists for at least one of the first and second load cells based upon information from the detection circuit.
 18. The person support apparatus of claim 17 wherein the error state includes an electrical disconnection of the first load cell from the controller.
 19. The person support apparatus of claim 17 further including an activation voltage source coupled to endpoints of the first and second Wheatstone bridges, wherein the detection circuit monitors a total amount of electrical current flowing from the activation voltage source to the endpoints of the first and second Wheatstone bridges.
 20. The person support apparatus of claim 15 wherein the controller stores first calibration data for the first load cell and second calibration data for the second load cell in a memory on board the person support apparatus. 